1. Field
The invention relates to communication systems. More particularly, the invention relates to methods and apparatus for scheduling or assigning resources such as rate and power in a wireless communication system.
2. Background
Several multiple access communication techniques are known in the art, such as time division multiple access (TDMA) and frequency division multiple access (FDMA). However, the spread spectrum modulation techniques of code division multiple access (CDMA) provide significant advantages over other multiple access modulation techniques. CDMA techniques in a communication system are disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
Since CDMA employs a wideband signal, it spreads the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. CDMA also provides space or path diversity through multiple signal paths that simultaneously link a mobile station or user with two or more cell-sites. Furthermore, CDMA can exploit the multipath environment by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
CDMA modulation techniques require that all transmitters be under precise power control to manage interference in the system. If the transmission power of signals transmitted by a base station to a user (the forward link) are too high, it can create problems such as interfering with other users. Most base stations have a fixed amount of power at which to transmit signals, and therefore can transmit to only a limited number of users. Alternatively, if the transmission power of signals transmitted by the base station is too low, then some users can receive multiple erroneous transmitted frames. Terrestrial channel fading and other known factors also affect the transmission power of signals transmitted by the base station. Thus, each base station needs to adjust the transmission power of the signals it transmits to its users. A method and apparatus for controlling transmission power is disclosed in U.S. Pat. No. 5,056,109, entitled “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention.
Under one CDMA standard, described in the Telecommunications Industry Association's TIA/EIA/IS-95-A Mobile Stations-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, each base station transmits pilot, sync, paging and forward traffic channels to its users. Under this standard, power control signals or codes are also exchanged between each base station and the mobile stations to provide appropriate power control for the system.
Improvements to the above standard have included additional, higher data rates. These higher data rates help provide for data services beyond traditional voice services. Voice services typically tolerate higher error rates than data services (e.g., a maximum bit error rate (BER) of 10−3), but require continuous bit stream transmissions with no delays. Most data, such as electronic mail, facsimile and general computer data, may use discontinuous, packetized data transmissions. Such data typically must be transmitted at bit rates higher than speech, but are insensitive to delay and require lower error rates. For example, facsimile, general computer data and email typically are transmitted at bit rates of 8-32 kbps, 0.1-1 Mbps, and 9.6-128 kbps, and maximum BER's of 10−4, 10−9 and 10−9, all respectively. Video requires even higher bit rates and lower error rates than voice, and, like voice, requires continuous bit stream transmissions. For example, low resolution video typically requires a bit rate of 64-128 kbps and a maximum BER of 10−5.
To be efficient, a wireless communication system must not provide the same data rate, error rate and bit stream (power) for all services based on the most stringent requirements of any one service. Therefore, one prior technique employs dynamic control algorithms for admission or registration control, resource allocation and error recovery and at burst or packet levels for a given base station. See, e.g., A. Sampath, P. Kumar and J. Holtzman, “Power Control and Resource Management for a Multimedia CDMA Wireless System,” PIMRC, 1995. Such a system, however, may provide ad hoc or immediate service allocation, which is not efficient or optimized. Each new service request is allocated at that time by the base station. Additionally, while one base station may optimize itself for an immediate service allocation, such optimizations may well create interference for adjacent base stations. If one base station is optimizing itself, interference it receives from an adjacent base station, (which is itself optimizing) can cause two adjacent base stations to continually create interference for each other and thereby result in an unstable condition within the wireless communication system.